A closing device of this type, for example, is described in DE-OS 36 19 648. On closure of binding combs the open form is generally closed by a bending or rolling-in operation following previous insertion of the appropriate preperforated single sheets into the perforating members. During the closing operation the perforating members of the binding comb slide on the bending punch or dies. The sliding depends on a large number of factors, these impinging both on the top side opposite the top bending punch or die and on the underside opposite the bottom bending punch or die. It would be desirable, however, to have sliding and a resultant roll-in movement as far as possible only of the perforating members lying on the bottom of the bending punch or die, whereas the position of the perforating members opposite the top bending punch or die should be retained as far as possible. In practice the roll-in movement takes place more or less in an uncontrolled manner, with the top perforating members rolling themselves in pressing on the pile of sheets and in certain circumstances producing undesired impressions in the uppermost sheet.
A further disadvantage with the well-known closing devices is that several bending punches or dies are necessary for the binding of different numbers of sheets and of binding combs with different diameters. In practice eleven pairs of bending punches or dies are generally required for the sizes currently on the market. This large number of bending punches or dies represents both a problem regarding warehousing or stocking and regarding costs.
The problem of sliding conditions of the binding comb during closure is discussed in the previously mentioned DE-OS 36 19 648. An attempt was made to influence the sliding conditions by a special embodiment of at least a part of the sliding surface of the working punch or die with rolling elements or components so that binding combs were closed in a repeatable manner in a constant pre-determined profile form. The solution described, however, is relatively expensive and also successful only to a certain extent.